Coding challenge: a sliding window map. Quick dummy impl on Java.

SlidingWindowMap.java

import java.util.Date;
import java.util.Iterator;
import java.util.Set;

/**
 * @author Stanislav Kurilin
 */
public class SlidingWindowMap {
    private final Set<String> keys;
    private final int maxCount;
    private final long periodMs;

    private long periodStart;
    private int countOnPeriod;
    private Iterator<String> it;
    private String currentKey;

    public SlidingWindowMap(Set<String> keys, int maxCount, long periodMs) {
        this.keys = keys;
        this.maxCount = maxCount;
        this.periodMs = periodMs;
        periodStart = current();
        it = keys.iterator();
    }

    /**
     * @return a key that has been used less than `maxCount` times during the
     *         past `periodMs` milliseconds or null if no such key exists.
     */
    public String getNextKey() {
        if (timeToStartNewPeriod()) beginNewPeriod();
        if (currentKeyIsValid()) return currentKey();
        if (thereAreValidKeys()) return newValidKey();
        return null;
    }

    /**
     * @return current ms
     */
    private long current() {
        return new Date().getTime();
    }

    /**
     * @return true if new period should begin false otherwise
     */
    private boolean timeToStartNewPeriod() {
        return current() - periodStart > periodMs;
    }

    /**
     * reinitialize counters
     */
    private void beginNewPeriod() {
        periodStart = current();
        countOnPeriod = 0;
        it = keys.iterator();
    }

    /**
     * @return true if current key steel valid
     */
    private boolean currentKeyIsValid() {
        return countOnPeriod < maxCount;
    }

    /**
     * @return current key
     */
    private String currentKey() {
        countOnPeriod++;
        return currentKey;
    }

    /**
     * @return true if there are valid keys available
     */
    private boolean thereAreValidKeys() {
        return it.hasNext();
    }

    /**
     * @return new valid key
     * @throws java.util.NoSuchElementException if not available
     */
    private String newValidKey() {
        currentKey = it.next();
        return currentKey;
    }
}